Lost Administrator privileges on MacBook pro

I have lost my administrator privileges on my MacBook Pro and I am now a 'standard' user. I am unable to make any changes via System Preferences to this because I am prompted with user name and password to approve the changes but I get bounced out as I am not and administrator of the computer anymore. Any ideas? I tried to reboot the device but with no success, maybe I did something wrong.

Please help me! I need Mac for work and I am in Italy in emergency period...